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Annals of Mathematics The diameter of the isomorphism class of a Banach space By W. B. Johnson and E. Odell Annals of Mathematics, 162 (2005), 423–437 The diameter of the isomorphism class of a Banach space By W. B. Johnson and E. Odell* Dedicated to the memory of V. I. Gurarii Abstract We prove that if X is a separable infinite dimensional Banach space then its isomorphism class has infinite diameter with respect to the Banach-Mazur distance. One step in the proof is to show that if X is elastic then X contains an isomorph of c 0 . We call X elastic if for some K<∞ for every Banach space Y which embeds into X, the space Y is K-isomorphic to a subspace of X. We also prove that if X is a separable Banach space such that for some K<∞ every isomorph of X is K-elastic then X is finite dimensional. 1. Introduction Given a Banach space X, let D(X) be the diameter in the Banach-Mazur distance of the class of all Banach spaces which are isomorphic to X; that is, D(X) = sup{d(X 1 ,X 2 ):X 1 ,X 2 are isomorphic to X} where d(X 1 ,X 2 ) is the infimum over all isomorphisms T from X 1 onto X 2 of T ·T −1 . It is well known that if X is finite (say, N) dimensional, then cN ≤ D(X) ≤ N for some positive constant c which is independent of N. The upper bound is an immediate consequence of the classical result (see e.g. [T-J, p. 54]) that d(Y, N 2 ) ≤ √ N for every N dimensional space Y . The lower bound is due to Gluskin [G], [T-J, p. 283]. It is natural to conjecture that D(X) must be infinite when X is infinite dimensional, but this problem remains open. As far as we know, this problem was first raised in print in the 1976 book of J. J. Sch¨affer [S, p. 99]. The problem was recently brought to the attention of the authors by V. I. Gurarii, who *Johnson was supported in part by NSF DMS-0200690, Texas Advanced Research Pro- gram 010366-163, and the U.S Israel Binational Science Foundation. Odell was supported in part by NSF DMS-0099366 and was a participant in the NSF supported Workshop in Linear Analysis and Probability at Texas A&M University. 424 W. B. JOHNSON AND E. ODELL checked that every infinite dimensional super-reflexive space as well as each of the common classical Banach spaces has an isomorphism class whose diameter is infinite. To see these cases, note that if X is infinite dimensional and E is any finite dimensional space, then it is clear that X is isomorphic to E⊕ 2 X 0 for some space X 0 . Therefore, if D(X) is finite, then X is finitely complementably universal; that is, there is a constant C so that every finite dimensional space is C-isomorphic to a C-complemented subspace of X. This implies that X cannot have nontrivial type or nontrivial cotype or local unconditional structure or numerous other structures. In particular, X cannot be any of the classical spaces or be super-reflexive. In his unpublished 1968 thesis [Mc], McGuigan conjectured that D(X) must be larger than one when dim X>1. Sch¨affer [S, p. 99] derived that D(X) ≥ 6/5 when dim X>1 as a consequence of other geometrical results contained in [S], but one can prove directly that D(X) ≥ √ 2. Indeed, it is clearly enough to get an appropriate lower bound on the Banach-Mazur dis- tance between X 1 := Y ⊕ 1 R and X 2 := Y ⊕ 2 R when Y is a nonzero Banach space. Now X 1 has a one dimensional subspace for which every two dimen- sional superspace is isometric to  2 1 . On the other hand, every one dimensional subspace of X 2 is contained in a two dimensional superspace which is isometric to  2 2 . It follows that d(X 1 ,X 2 ) ≥ d( 2 1 , 2 2 )= √ 2. The Main Theorem in this paper is a solution to Sch¨affer’s problem for separable Banach spaces: Main Theorem. If X is a separable infinite dimensional Banach space, then D(X)=∞. Part of the work for proving the Main Theorem involves showing that if X is separable and D(X) < ∞, then X contains an isomorph of c 0 . This proof is inherently not local in nature, and, strangely enough, local considerations, such as those mentioned earlier which yield partial results, play no role in our proof. We do not see how to prove that a nonseparable space X for which D(X) < ∞ must contain an isomorph of c 0 . Our proof requires Bourgain’s index theory which in turn requires separability. Our method of proof involves the concept of an elastic Banach space. Say that X is K-elastic provided that if a Banach space Y embeds into X then Y must K-embed into X (that is, there is an isomorphism T from Y into X with y≤Ty≤Ky for all y ∈ Y ). This is the same (by Lemma 2) as saying that every space isomorphic to X must K-embed into X. X is said to be elastic if it is K-elastic for some K<∞. Obviously, if D(X) < ∞ then X as well as every isomorph of X is D(X)- elastic. Thus the Main Theorem is an immediate consequence of THE ISOMORPHISM CLASS OF A BANACH SPACE 425 Theorem 1. If X is a separable Banach space and there is a K so that every isomorph of X is K-elastic, then X is finite dimensional. A key step in our argument involves showing that an elastic space X admits a normalized weakly null sequence having a spreading model not equiv- alent to either the unit vector basis of c 0 or  1 . To achieve this we first prove (Theorem 7) that if X is elastic then c 0 embeds into X. It is reasonable to conjecture that an elastic infinite dimensional separable Banach space must contain an isomorph of C[0, 1]. Theorem 1 would be an immediate conse- quence of this conjecture and the “arbitrary distortability” of C[0, 1] proved in [LP]. Our derivation of Theorem 1 from Theorem 7 uses ideas from [LP] as well as [MR]. With the letters X, Y, Z, . we will denote separable infinite dimensional real Banach spaces unless otherwise indicated. Y ⊆ X will mean that Y is a closed (infinite dimensional) subspace of X. The closed linear span of the set A is denoted [A]. We use standard Banach space theory terminology, as can be found in [LT]. The material we use on spreading models can be found in [BL]. For simplicity we assume real scalars, but all proofs can easily be adapted for complex Banach spaces. 2. The main result The following lemma [Pe] shows that the two definitions of elastic men- tioned in Section 1 are equivalent. Lemma 2. Let Y ⊆ (X, ·) and let |·| be an equivalent norm on (Y,·). Then |·|can be extended to an equivalent norm on X. Proof. There exist positive reals C and d with dy≤|y|≤Cy for y ∈ Y . Let F ⊆ CB X ∗ be a set of Hahn-Banach extensions of all elements of S (Y ∗ ,|·|) to all of X.Forx ∈ X define |x| = sup  |f(x)| : f ∈ F  ∨ dx . Let n ∈ N and K<∞. We shall call a basic sequence (x i ) block n-unconditional with constant K if every block basis (y i ) n i=1 of (x i )isK- unconditional; that is,  n  i=1 ±a i y i ≤K n  i=1 a i y i  for all scalars (a i ) n i=1 and all choices of ±. The next lemma is essentially contained in [LP]. In fact, by using the slightly more involved argument in [LP], the conclusion “with constant 2” can 426 W. B. JOHNSON AND E. ODELL be changed to “with constant 1 + ε”, which implies that the constant in the conclusion of Lemma 4 can be changed from 2 + ε to 1 + ε. Lemma 3. Let X be a Banach space with a basis (x i ). For every n there is an equivalent norm |·| n on X so that in (X, |·| n ), (x i ) is block n-unconditional with constant 2. Proof. Let (P n ) be the sequence of basis projections associated with (x n ). We may assume, by passing to an equivalent norm on X, that (x n ) is bimono- tone and hence P j − P i  = 1 for all i<j. Let S n be the class of operators S on X of the form S =  m k=1 (−1) k (P n k −P n k−1 ) where 0 ≤ n 0 < ···<n m and m ≤ n. Define |x| n := sup{Sx : S ∈S n } . Thus x≤|x| n ≤ nx for x ∈ X. It suffices to show that for S ∈S n , |S| n ≤ 2. Let x ∈ span (x n ) and |Sx| n = TSx for some T ∈ S n . Then since TS ∈S 2n ⊆S n + S n , TSx≤2|x| n . Lemma 4. For every separable Banach space X and n ∈ N there exists an equivalent norm |·|on X so that for every ε>0, every normalized weakly null sequence in X admits a block n-unconditional subsequence with constant 2+ε. Proof. Since C[0, 1] has a basis, the lemma follows from Lemma 3 and the the classical fact that every separable Banach space 1-embeds into C[0, 1]. Lemma 4 is false for some nonseparable spaces. Partington [P] and Talagrand [T] proved that every isomorph of  ∞ contains, for every ε>0, a1+ε-isometric copy of  ∞ and hence of every separable Banach space. Our next lemma is an extension of the Maurey-Rosenthal construction [MR], or rather the footnote to it given by one of the authors (Example 3 in [MR]). We first recall the construction of spreading models.If(y n )isa normalized basic sequence then, given ε n ↓ 0, one can use Ramsey’s theorem and a diagonal argument to find a subsequence (x n )of(y n ) with the following property. For all m in N and (a i ) m i=1 ⊂ [−1, 1], if m ≤ i 1 < ··· <i m and m ≤ j 1 < ···<j m , then         m  k=1 a k x i k    −    m  k=1 a k x j k         <ε m . It follows that for all m and (a i ) m i=1 ⊂ R, lim i 1 →∞ lim i m →∞    m  k=1 a k x i k    ≡    m  k=1 a k ˜x k    THE ISOMORPHISM CLASS OF A BANACH SPACE 427 exists. The sequence (˜x i ) is then a basis for the completion of (span (˜x i ), ·) and (˜x i ) is called a spreading model of (x i ). If (x i ) is weakly null, then (˜x i ) is 2-unconditional. One shows this by checking that (˜x i ) is suppression 1-unconditional, which means that for all scalars (a i ) m i=1 and F ⊂{1, ,m},       i∈F a i ˜x i      ≤      m  i=1 a i ˜x i      . Also, (x i ) is 1-spreading, which means that for all scalars (a i ) m i=1 and all n(1) < ···<n(m),      m  i=1 a i ˜x i      =      m  i=1 a i ˜x n(i)      . It is not difficult to see that, when (x i ) is weakly null, (˜x i ) is not equivalent to the unit vector basis of c 0 (respectively,  1 ) if and only if lim m   m i=1 ˜x i  = ∞ (respectively, lim m   m i=1 ˜x i /m = 0). All of these facts can be found in [BL]. Lemma 5. Let (x n ) be a normalized weakly null basic sequence with spread- ing model (˜x n ). Assume that (˜x n ) is not equivalent to either the unit vector basis of  1 or the unit vector basis of c 0 . Then for all C<∞ there exist n ∈ N, a subsequence (y i ) of (x i ), and an equivalent norm |·|on [(y i )] so that (y i ) is |·|-normalized and no subsequence of (y i ) is block n-unconditional with constant C for the norm |·|. Proof. Recall that if (e i ) n 1 is normalized and 1-spreading and bimonotone then   n 1 e i   n 1 e ∗ i ≤2n where (e ∗ i ) n 1 is biorthogonal to (e i ) n 1 [LT, p. 118]. Thus e =  n 1 e i   n 1 e i  is normed by f =   n 1 e i  n  n i=1 e ∗ i , precisely f(e)=1=e, and f ≤2. These facts allow us to deduce that there is a subsequence (y i ) of (x i ) so that if F ⊆ N is admissible (that is, |F |≤min F ) then f F ≡   i∈F y i  |F |   i∈F y ∗ i  satisfies f F ≤5 and f F (y F ) = 1, where y F ≡  i∈F y i   i∈F y i  . Indeed, (˜x i ) is 1-spreading and suppression 1-unconditional (since (x i ) is weakly null). Given 1/2 >ε>0 we can find (y i ) ⊆ (x i ) so that if F ⊆ N is admissible then (y i ) i∈F is 1+ε-equivalent to (˜x i ) |F | i=1 . Furthermore we can choose (y i ) so that if F is admissible then for y =  a i y i ,   i∈F a i y i ≤ (2 + ε)y (for a proof see [O] or [BL]). Hence f F ≤(2 + ε)f F | [y i ] i∈F  < 5 for sufficiently small ε by our above remarks. We are ready to produce a Maurey-Rosenthal type renorming. Choose n so that n>7C and let ε>0 satisfy n 2 ε<1. We choose a subsequence 428 W. B. JOHNSON AND E. ODELL M =(m j ) ∞ j=1 of N so that m 1 = 1 and for i = j and for all admissible sets F and G with |F | = m i and |G| = m j , a)   k∈F y k    k∈G y k  <ε,ifm i <m j and b)   k∈F y k    k∈G y k  m j m i <ε,ifm i >m j . Indeed, we have chosen (y i ) so that 1 2    |F |  i=1 ˜x i    ≤     i∈F y i    ≤ 2    |F |  i=1 ˜x i    and similarly for G. Since (˜x i ) is not equivalent to the unit vector basis of c 0 (and is unconditional) lim m   m 1 ˜x i  = ∞ so that a) will be satisfied if (m k ) increases sufficiently rapidly. Furthermore, since (˜x i ) is not equivalent to the unit vector basis of  1 , lim m   m 1 ˜x i  m = 0 and so b) can also be achieved. For i ∈ N set A i = {y F : F is admissible and |F | = m i } and A ∗ i = {f F : F is admissible and |F| = m i }. Let φ be an injection into M from the collection of all (F 1 , ,F i ) where i<nand F 1 <F 2 < ···<F i are finite subsets of N. Here F<Gmeans max F<min G. Let F =  n  i=1 f F i : F 1 < ···<F n , |F 1 | = m 1 =1, each F i is admissible and |F i+1 | = φ(F 1 , ,F i ) for 1 ≤ i<n  . For y ∈ [(y i )] let y F = sup  |f(y)| : f ∈F  and set |y| = y F ∨ εy . This is an equivalent norm since for f ∈F, f≤5n. Also, |y i | = 1 for all i. Note that if f F ∈A ∗ i and y G ∈A j with i = j then |f F (y G )| =   k∈F y k  m i  k∈F y ∗ k   k∈G y k   k∈G y k   ≤   k∈F y k    k∈G y k  m i ∧ m j m i . If m i <m j then |f F (y G )| <εby a). If m i >m j then |f F (y G )| <εby b). It follows that if y =  n i=1 y F i and f =  n i=1 f F i ∈Fthen |y|≥f(y)=n and if z =  n i=1 (−1) i y F i , then for all g =  n j=1 f G j ∈F, |g(z)|≤6+n 2 ε<7. Indeed, we may assume that g = f and if F 1 = G 1 then |G j | = |F i | for all 1 <i≤ n and 1 ≤ j ≤ n and so by a), b), |g(z)| =     n  j=1 f G j  n  i=1 (−1) i y F i      ≤ n  j=1 n  i=1 |f G j (y F i )| <n 2 ε. THE ISOMORPHISM CLASS OF A BANACH SPACE 429 Otherwise there exists 1 ≤ j 0 <nso that F j = G j for j ≤ j 0 , |F j 0 +1 | = |G j 0 +1 | and |F i | = |G j | for j 0 +1<i,j≤ n. Using f G j 0 +1 (z) ≤ 5+nε we obtain |g(z)|≤     j 0  j=1 f G j (z)     + |f G j 0 +1 (z)| +     n  j>j 0 +1 f G j (z)     < 1+5+nε +(n −(j 0 + 1))nε < 6+n 2 ε. Hence |z|≤7 follows and the lemma is proved since n/7 >Cand such vectors y and z can be produced in any subsequence of (y i ). Our next lemma follows from Proposition 3.2 in [AOST]. Lemma 6. Let X be a Banach space. Assume that for all n,(x n i ) ∞ i=1 is a normalized weakly null sequence in X having spreading model (˜x n i ) which is not equivalent to the unit vector basis of  1 . Then there exists a normalized weakly null sequence (y i ) ⊆ X with spreading model (˜y i ) such that (˜y i ) is not equivalent to the unit vector basis of  1 . Moreover, there exists λ>0 so that for all n, λ2 −n   a i ˜x n i ≤  a i ˜y i  for all (a i ) ⊆ R. Theorem 7. Let X be elastic, separable and infinite dimensional. Then c 0 embeds into X. We postpone the proof to complete first the Proof of Theorem 1. Assume that X is infinite dimensional and every isomorph of X is K-elastic. Then by Theorem 7, c 0 embeds into X. Choose k n ↑∞so that 2 −n k n →∞. Using the renormings of c 0 by |(a i )| n = sup      F a i    : |F | = k n  and that X is K-elastic we can find for all n a normalized weakly null sequence (x n i ) ∞ i=1 ⊆ X with spreading model (˜x n i ) ∞ i=1 satisfying   a i ˜x n i ≥K −1 |(a i )| n and moreover each (˜x n i ) is equivalent to the unit vector basis of c 0 .Thus by Lemma 6 there exists a normalized weakly null sequence (y i )inX having spreading model (˜y i ) which is not equivalent to the unit vector basis of  1 and which satisfies for all n,     k n  1 ˜y i     ≥ λK −1 2 −n k n →∞. Thus (˜y i ) is not equivalent to the unit vector basis of c 0 as well. 430 W. B. JOHNSON AND E. ODELL By Lemmas 2 and 5, for all C<∞ we can find n ∈ N and a renorming Y of X so that Y contains a normalized weakly null sequence admitting no sub- sequence which is block n-unconditional with constant C. By the assumption on X, the space Y must K-embed into every isomorph of X. But if C is large enough this contradicts Lemma 4. It remains to prove Theorem 7. We shall employ an index argument involving  ∞ -trees defined on Banach spaces. If Y is a Banach space our trees T on Y will be countable. For some C the nodes of T will be elements (y i ) n 1 ⊆ Y with (y i ) n 1 bimonotone basic and satisfying 1 ≤y i  and   n 1 ±y i ≤C for all choices of sign. Thus (y i ) n 1 is C-equivalent to the unit vector basis of  n ∞ . T is partially ordered by (x i ) n 1 ≤ (y i ) m 1 if n ≤ m and x i = y i for i ≤ n. The order o(T ) is given as follows. If T is not well founded (i.e., T has an infinite branch), then o(T )=ω 1 . Otherwise we set for such a tree S, S  = {(x i ) n 1 ∈ S :(x i ) n 1 is not a maximal node}. Set T 0 = T , T 1 = T  and in general T α+1 =(T α )  and T α = ∩ β<α T β if α is a limit ordinal. Then o(T ) = inf{α : T α = φ} . By Bourgain’s index theory [B1], [B2] (see also [AGR]), if X is separable and contains for all β<ω 1 such a tree of index at least β, then c 0 embeds into X. We now complete the Proof of Theorem 7. Without loss of generality we may assume that X ⊆ Z where Z has a bimonotone basis (z i ). Let X be K-elastic. We will often use semi-normalized sequences in X which are a tiny perturbation of a block basis of (z i ) and to simplify the estimates we will assume below that they are in fact a block basis of (z i ). For example, if (y i ) is a normalized basic sequence in X then we call (d i ) a difference sequence of (y i )ifd i = y k(2i) −y k(2i+1) for some k 1 <k 2 < ···.We can always choose such a (d i ) to be a semi-normalized perturbation of a block basis of (z i ) by first passing to a subsequence (y  i )of(y i ) so that lim i→∞ z ∗ j (y  i ) exists for all j, where (z ∗ i ) is biorthogonal to (z i ), and taking (d i )tobea suitable difference sequence of (y  i ). We will assume then that (d i ) is in fact a block basis of (z i ). We inductively construct for each limit ordinal β<ω 1 , a Banach space Y β that embeds into X. Y β will have a normalized bimonotone basis (y β i ) that can be enumerated as (y β i ) ∞ i=1 = {y β,ρ,n i : ρ ∈ C β , n ∈ N, i ∈ N} where C β is some countable set. The order is such that (y β,ρ,n i ) ∞ i=1 is a subsequence of (y β i ) for fixed ρ and n. Before stating the remaining properties of (y β i ) we need some terminology. We say that (w i )isacompatible difference sequence of (y β i ) of order 1if(w i ) THE ISOMORPHISM CLASS OF A BANACH SPACE 431 is a difference sequence of (y β i ) that can be enumerated as follows, (w i )={w β,ρ,n i : ρ ∈ C β ,n,i∈ N} and such that for fixed ρ and n, (w β,ρ,n i ) i is a difference sequence of (y β,ρ,n+1 i ) i . If (v i ) is a compatible difference sequence of (w i ) of order 1, in the above sense, (v i ) will be called a compatible difference sequence of (y β i ) of order 2, and so on. (y β i ) will be said to have order 0. Let (v i ) be a compatible difference sequence of (y β i ) of some finite order. We set T  (v i )  =  (u i ) s 1 : the u i ’s are distinct elements of {v i } ∞ 1 , possibly in different order, and     s  1 ±u i     = 1 for all choices of sign  . T  (v i )  is then an  ∞ -tree as described above with C = 1. The inductive con- dition on Y β , or should we say on (y β,ρ,n i ), is that for all compatible difference sequences (v i )of(y β,ρ,n i ) of finite order, o(T ((v i ))) ≥ β. Before proceeding we have an elementary but key Sublemma. Let C<∞ and let (w i ) be a block basis of a bimonotone basis (z i ) with 1 ≤w i ≤C for all i and let A = {F ⊆ N : F is finite and   i∈F ±w i ≤C for all choices of sign}. Then there exists an equivalent norm |·|on [(w i )] so that (w i ) is a bimonotone normalized basis such that for all F ∈A,     F ±w i    =1. Proof. Define |  a i w i | = (a i ) ∞ ∨ C −1   a i w i . We begin by constructing Y ω . Let (x i ) ⊆ X be a normalized block basis of (z i ). For n ∈ N, let |·| n be an equivalent norm on [(x i )] given by the sublemma for C =2 n .Thus|  F ±x i | n =1if|F |≤2 n . Since X is K-elastic, for all n, ([(x i )], |·| n ) K-embeds into X.Wethus obtain for n ∈ N, a sequence (x n i ) i ⊆ X with 1 ≤x n i ≤K for all i and such that 1 ≤  i∈F ±x n i ≤K for all |F |≤2 n and all choices of sign. Furthermore [...]... Bourgain’s Math 108 (1998), 145–171 [LP] ´ J Lindenstrauss and A Pelczynski, Contributions to the theory of the classical Banach spaces, J Funct Anal 8 (1971), 225–249 [LT] J Lindenstrauss and L Tzafriri, Classical Banach Spaces I Sequence Spaces, 1 -index of a Banach space, Israel J Springer-Verlag New York (1977) [Mc] R A McGuigan, Jr., Near isometry of Banach spaces and the Banach- Mazur dis- tance,... Does there exist a nonseparable Banach space X and a K < ∞ so that all isomorphs of X are K-elastic? Under the generalized continuum hypothesis, it follows from [E] that for every cardinal ℵ there is a Banach space X of density character ℵ so that every Banach space of density character ℵ is isometrically isomorphic to a subspace of X These provide the only known examples of elastic Banach spaces Problem... College Station, TX E-mail address: johnson@math.tamu.edu The University of Texas at Austin, Austin, TX E-mail address: odell@math.utexas.edu References [A] I Aharoni, Every separable metric space is Lipschitz equivalent to a subset of c+ , 0 Israel J Math 19 (1974), 284–291 [AGR] S Argyros, G Godefroy, and H Rosenthal, Descriptive set theory and Banach spaces, in Handbook of the Geometry of Banach Spaces,... dimensional Banach space and there is a K so that every isomorph of X is Lipschitz K-elastic, must X contain a subspace isomorphic to c0 ? We also do not know whether the Main Theorem has a Lipschitz analogue: Problem 11 If X is a separable Banach space and there exists K < ∞ so that dL (X, Y ) < K for all spaces Y which are isomorphic (or biLipschitz equivalent) to X, then must X be finite dimensional? The. .. Johnson and J Lindenstrauss, eds.) North-Holland, Amsterdam (2003), 1007–1069 [AOST] G Androulakis, E Odell, Th Schlumprecht, and N Tomczak-Jaegermann, On the structure of the spreading models of a Banach space, Canadian J Math 57 (2005), 673–707 ´ [BL] B Beauzamy and J.-T Lapreste, Modeles Etal´s des Espaces de Banach, in Travaux ` ´ e en Cours, Hermann, Paris, 1984 [BeLi] Y Benyamini and J Lindenstrauss,... space has, for each > 0, Lipschitz distance less than 3 + to some subset of c0 , while James [J], [BeLi, Proposition 13.6] proved that for each > 0, the space c0 is 1 + -equivalent to a subspace of every isomorph of c0 Consequently, any separable Banach space which contains an isomorphic copy of c0 is Lipschitz 3 + -elastic THE ISOMORPHISM CLASS OF A BANACH SPACE 435 Problem 10 If X is a separable,... K-elastic Are there other examples of separable elastic spaces? Problem 8 Let X be elastic (and separable, say) Does C[0, 1] embed into X? Using index arguments, we have the following partial result Proposition 9 Let X be a separable Banach space, and suppose that Y = Xn is a symmetric decomposition of a space Y into spaces uniformly isomorphic to X If Y is elastic, then C[0, 1] embeds into X In particular,... THE ISOMORPHISM CLASS OF A BANACH SPACE [Pe] 437 ´ A Pelczynski, Projections in certain Banach spaces, Studia Math 19 (1960), 209– 228 [R] H P Rosenthal, On factors of C([0, 1]) with non-separable dual, Israel J Math 13 (1972), 361–378 [S] ¨ J J Schaffer, Geometry of Spheres in Normed Spaces, Lecture Notes Pure Appl Math 20, Marcel Dekker, Inc., New York (1976) [T] M Talagrand, Sur les espaces de Banach. .. of the empty set is ∞) Call a Banach space X Lipschitz K-elastic provided that every isomorph of X has Lipschitz distance at most K to some subset of X, and say that X is Lipschitz elastic if X is Lipschitz K-elastic for some K < ∞ It is interesting to note that the analogue of Theorem 1 for Lipschitz elasticity is false Indeed, Aharoni [A] , [BeLi, Theorem 7.11] proved that every separable metric space. .. If X is elastic and infinite dimensional, does X contain an isomorph of every Banach space whose density character is the same as the density character of X? As was mentioned earlier, the space ∞ is, for every > 0, 1+ -isomorphic to a subspace of every isomorph of itself (see [P] and [T]) However, ∞ is not elastic To see this, take a biorthogonal system (xα , x∗ )α . Annals of Mathematics The diameter of the isomorphism class of a Banach space By W. B. Johnson and E. Odell Annals of Mathematics,. K-elastic then X is finite dimensional. 1. Introduction Given a Banach space X, let D(X) be the diameter in the Banach- Mazur distance of the class of all Banach